The present invention relates to a method of the formation of an image and an apparatus used therefor, and more particularly to a method and apparatus, e.g., an image-reading method and apparatus or image-reproducing method and apparatus, which are capable of processing by separating and synthesizing in a simple manner a color image consisting of chromatic and achromatic color components or of different chromatic color components.
As the process for the formation of a color image, there are conventionally known processes for the image signal processing such as, for example, the color printing process, electrophotographic process, color scanner process, and the like. Any of these processes requires for the image formation the obtaining of pieces of information of three colors by separating the original image color into three primary colors (blue (B), green (G) and red (R)). For example, according to the prior-art full-color-process electrophotographic copier, a photoreceptor, after being corona-charged, is exposed through a red filter to the light from an original and then developed by a cyan developer, and the resulting visible cyan image is once transferred onto a copying paper. Next, in like manner, the photoreceptor is exposed through a green filter and developed by a magenta developer, and the formed visible magenta image is then transferred with registering onto the above cyan image on the same copying paper. Further, the same manner is repeated with use of a blue filter and yellow developer to form a visible yellow image registered on the foregoing two-color image on the same copying paper. And the three-color-superposed image, if necessary, is fixed, whereby a final color image is obtained.
On the other hand, the image formation based on two-color information is known as a masking technique, by the color correcting method used in the color printing process. According to the positive masking technique, color correction is performed in each color printing plate making process by superposing upon an uncorrected color-separated negative image a required density-having color-separated positive image prepared from a different color-separated negative.
The multicolor copying method include a method which uses an NP photoreceptor. For example, Japanese Patent Examined Publication No. 34770/1973 discloses a method in which a photoreceptor is exposed imagewise through a red filter and at the same time positively charged, then exposed imagewise through a cyan filter and simultaneously negatively charged or A.C.-charged to form thereon an electrostatic latent image for red color only, and then, after the latent image's red-color development and transfer onto a copying paper, is further subjected to another electrostatic latent image formation for black only in usual manner, which is then developed to a black image to be transferred onto the same copying paper. Also, Japanese Patent Examined Publication No. 33066/1980 discloses a method which provides a two-color image in the manner that a photoreceptor is subjected to an overall exposure and positive charging at the same time, then to an imagewise exposure through a cyan filter and negative or A.C. charging at the same time, and further to an imagewise exposure through a red filter to thereby form electrostatic latent images for red and black to be charged to opposite polarities to each other, and the respective latent images are developed with opposite-polarity toners to each other to form a two-color image to be transferred onto a copying paper.
However, in the method using the above NP photoreceptor, the simultaneous performing of the charging and exposure is essential for its process, but it is not easy to control both exposed and unexposed areas so as to be charged under the same condition because of the difference in the condition of the photoreceptor between the exposed and unexposed areas. Moreover, since the imagewise exposure and charging are performed simultaneously it is relatively difficult to maintain the charged potential sufficiently. In the process using the NP photoreceptor, in the case of the double-imagewise exposure process, it is difficult to eliminate the black image (particularly fine lines), while in the case of the single imagewise exposure process, it is difficult to secure sufficiently the potentials of both black and red images. And in the case of the process using a photoreceptor of a large carrier transit time (e.g., an organic semiconductor), the photoreceptor's sensitivity tends to be deteriorated.
On the other hand, there is a different multicolor copying process which uses a composite photoreceptor comprised of a red-light-insensitive layer superposed on an at least red-light-sensitive layer, as disclosed in Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 3537/1979. According to this process, a primary charging is performed with uniform exposure so that the upper photosensitive layer alone becomes photoconductive, and then a secondary opposite-polarity charging is performed in the dark, and further an imagewise exposure is performed to thereby eliminate the charge in the corresponding region to the white image area and to leave the charge of the red-light-insensitive layer in only the corresponding region to the red image area, and thus its surface potential's polarity is made opposite to that of the black area.
In this method of the prior art, however, the construction of the photoreceptor is so complex that its preparation is difficult, and besides, the electric potential's stability during the repeated use of the photoreceptor is deteriorated by the influence of the above respective photosensitive layers' residual potentials, and the like.
Also, according to the known full-color-process electrophotographic copier, for example, the photoreceptor, after its corona charging, is exposed through a red filter to the light from an original image and developed by a cyan developed, and the thus obtained visible cyan image is once transferred onto a copying paper. Next, in like manner, the photoreceptor is exposed through a green filter, then developed by a magenta developer, and the resulting visible magenta image is then transferred in register with the above cyan image. Further, the same process is repeated with use of a blue filter and an yellow developer to form a visible yellow image, which is then transferred with registering onto the above two-color image, and then the formed three-color image, if necessary, is fixed, whereby the final color image is obtained.
There is a subtractive color image reproduction process similar to the above color process. This is known as the color scanner process.
In addition, there is also known a copying process which uses a photosensitive screen having a number of apertures (particularly fine-mesh apertures ) on which is formed an electrostatic image, by which is controlled the passage of the charge particles' flow (e.g., positive ionic particles) to thereby form a given electrostatic image on a chargeable layer (e.g., the photosensitive layer of the photoreceptor).
For the above photosensitive screen method, a NP screen photoreceptor disclosed in Japanese Patent Examined Publication No. 31376/1979, which is comprised of a photoconductive layer and an insulating layer provided in the described order on a screen base. An electrophotographic process which uses this NP screen photoreceptor, as disclosed in, e.g., Japanese Patent Examined Publication No. 27144/1979, comprises the application of a primary charging to the screen photoreceptor, the simultaneous application of a secondary charging (A.C. or D.C.) and imagewise exposure, and the application of an overall exposure in the described order.
A technique of an electrophotographic process to which is applied the masking technique used in the printing plate making process is disclosed in Japanese Patent O.P.I. Publication No. 3430/1977. According to this prior art, a first electrostatic image is formed on a photoreceptor, and a second electrostatic image is formed on a photosensitive screen. In accordance with this second electrostatic image, a charge flow of opposite polarity to that of the first electrostatic image is projected upon the first electrostatic image to thereby correct the same. By this process, for example, magenta color can be reproduced, but it is no more than a process only for color correction. Accordingly, it is not for the purpose of separating the chromatic color components from the achromatic components, and particularly chromatic color signal levels can not be separated into two groups to be formed on both sides of the achromatic color signal levels.
In those copying processes of the prior art, in order to obtain a black image, yellow, magenta and cyan colors are superposed, but the blackness reproducibility and registering accuracy are not satisfactory.